Today, most of automobile engines employ a fuel injection system to control an air-fuel ratio accurately for improving exhaust emissions or fuel consumption. In a fuel injection engine, an amount of fuel to be injected is determined by an amount of mass of intake air and a desired air-fuel ratio. In particular, with a multi-cylinder engine, an air flow meter is required to have the ability of measuring an amount of intake air for each cylinder with high accuracy and response. Air flow meters in common use for a fuel injection engine today or in the past are such as a vane type flow meter, a hot-wire flow meter, or a Karman vortices flow meter.
A vane type flow meter is not being used today for production engines because of the disadvantages such that a vane causes a large amount of flow resistance, and an inertia of vane lowers response under transient states of engine running, though, from the viewpoint of principle, it has the advantage of capability of measuring a mass flow rate.
A hot-wire flow meter has the advantage of the ability of measuring a mass flow rate of pulsation flow with high response, while the disadvantages of requiring some additional devices for detecting a flow direction when adopted for measurement in a flow field where both of the order and reverse direction flows occur alternately, and causing a measurement error by stains on a hot-wire surface. FIG. 17 shows an example of a multi-cylinder engine with a multi-point injection system employing a hot-wire flow meter. In the figure, referring number 19 refers to a cylinder, 13 refers to an intake valve, 17 refers to an exhaust valve, 12 refers to an intake pipe, 28 refers to conjunct exhaust manifold, 14 refers to an air collector, 15 refers to a throttle valve, 27 refers to an fuel injector, and 42 refers to a hot-wire flow meter. Intake air is adapted to pass through throttle valve 15 to be accumulated in air collector 14 after metered by hot-wire flow meter 42, and flow into cylinder 19 through intake pipe 12. Preferably, hot-wire flow meter 42 is installed on intake pipe 12 of each cylinder to measure an amount of intake air for each cylinder with high response, however, intake pipe 12 provides a flow field with poor conditions for measuring with a hot-wire sensor such as solid particulate in a back flow gas from a cylinder or fuel particles injected by injector 27 are floating, and inflow and back flow occur alternately. Accordingly, in most engines, hot-wire flow meter 42 is installed on an upstream section of air collector 14 that is in communication with intake pipe 12, and hence it is difficult to measure an amount of intake air for each cylinder or varying of air flow rate accurately when an engine is running in a transient state.
A Karman vortices flow meter has the advantages of high response to flow pulsation and high resistance to deterioration in measurement accuracy by stains on a sensor. However, there are the problems such that an output variable of the flow meter is related to a volumetric flow rate that must be converted to a mass flow rate for air-fuel ratio control application, hence a temperature sensor and a pressure sensor must be employed in combination with a flow sensor, some devices for detecting a flow direction is needed as well as a hot-wire flow meter, and further a design of an intake pipe is restricted because a calibration characteristics of an output of a sensor against a flow rate strongly depend on the designs of a cone to generate Karman vortices and a passage around the cone. Therefore, it is difficult to measure an amount of intake air for each cylinder or varying of intake air flow accurately when an engine is running in a transient state as well as a hot-wire flow meter.
Differential pressure type flow meters such as an orifice flow meter or a laminar flow meter are used for measuring an intake air flow rate in engine bench tests. However, they are not applied to an engine onboard air flow meter because of their disadvantages of lowering engine performances to cause a large amount of flow resistance from the viewpoint of principle, and poor applicability for measurement of pulsation flows like an engine intake flow such as a calibration characteristic of the orifice flow meter being in proportion to the square with an air flow rate, and low response to flow pulsation of the laminar flow meter, though they have the advantages of low cost of pressure sensors and high resistance to deterioration in measurement accuracy by stains.
There is the preceding unexamined patent publication No. 10-111159 titled ‘Source type mass flow meter’ that relates to a differential pressure type flow meter for measuring a mass flow rate of pulsating flow, however it is structurally complicated and costly employing a source flow generator supplying pulsating source flow to main flow to be measured.
Summarizing the descriptions in [0001] through [0006], requirements to flow meter for measuring an engine intake air flow rate are as the following.    1. Capability of measuring a mass flow rate for air-fuel ratio control application    2. Capability of measuring a pulsation flow in both the order and reverse direction with high response    3. A feature that flow resistance is not increased by a flow meter itself    4. Resistance to deterioration in an measurement accuracy by stains attributed to dusts or particles in the atmosphere or an EGR gas.    5. Capability of measuring an amount of air for each cylinder of multi-cylinder engine, being installed on each intake pipeHowever, no flow meter mentioned above meets all these requirements.